Oxides of Nitrogen, specifically NO and NO2 collectively referred to as NOx, are well known and toxic by-products of internal combustion engines e.g. motor vehicles, fossil fuel powered electricity generation systems and industrial processes. NOx, and more specifically NO, is formed via the reactions of free radicals in the combustion process, as first identified by Y. B. Zeldovich (Acta Physico-chern. USSR, 21 (1946) 577), viz:N2+O.→NO+N.  (1)N.+O2″NO+O.  (2)
As indicated Nitrogen Oxides are directly toxic to living beings (P. E. Morrow J. Toxicol Environ Health 13(2-3), (1984), 205-27), in addition NOx directly contributes to and is an indirect factor in several sources of environmental pollution. Thus Nitrogen oxides are directly involved in the formation of acid rain but are also reagents in the processes for the production of photochemical smog and ozone which have been correlated to significant adverse impacts on human health (M. V. Twigg, Applied Catalysis B, vol. 70, (2007), 2). Hence increasingly stringent legislative limits have been introduced in order to regulate the emission of such compounds from the exhausts of both gasoline and diesel internal combustion engines e.g. Euro 5 or Euro 6 [Regulation (EC) No 715/2007 of the European Parliament and of the Council, 20 Jun. 2007, Official Journal of the European Union L 171/1, see also Twigg, Applied Catalysis B, vol. 70, (2007), p 2-25 and R. M. Heck, R. J. Farrauto Applied Catalysis A vol. 221, (2001), p 443-457 and references therein].
The challenge for meeting the legislative NOx targets for stoichiometric gasoline engines is readily fulfilled by the application of the well established chemistry of the three way catalytic converter (e.g. see SAE 2005-01-1111). However the converse is true for NOx reduction for diesel compression ignition engines or other fuel lean i.e. oxygen rich combustion cycles, e.g. lean gasoline direct injection, since three-way catalytic conversion is only effective under stoichiometric air: fuel ratios (SAE 2005-01-1111). Thus while diesel/compression ignition engines may offer increased durability, provide high torque at low engine rpm, and increased fuel economy/decreased emissions their inherent lean burn operation provides a major challenge to fulfilling legislative NOx targets. Hence a range of exhaust after-treatment technologies have been developed to address this requirement. These technologies include, but are not limited to, engine control methodologies/modification, alternate combustion cycles and the use of after-treatment systems e.g. catalytic control devices which eliminate exhaust pollutants by promoting chemical changes to convert the unwanted NOx species into nitrogen. Currently technologies for NOx control include the Diesel NOx Trap/NOx Storage Catalyst (DNT/NSC), Urea/NH3 Selective Catalytic Reduction catalyst (SCR) and Hydrocarbon—SCR catalyst.
The chemistry of the Urea/Ammonia SCR catalyst comprises a complex set of decomposition (eqn 3—for Urea feed) and reduction—oxidation reactions (eqns 4-9) with diverse surface intermediates which form the basis for extensive academic and practical study e.g. App Cat B 13 (1997) 1-25, App Cat B 84 (2008) 497, J. Phys. Chem. C (2009), 113, 1393, SAE 2008-01-1184, SAE 2008-01-1323 etc. These reactions are summarised in eqns 3-9. Equations 4-6 detail the desired chemistries of the Selective Catalytic Reduction (SCR) catalyst i.e. the interaction between an oxidised form of Nitrogen (NO, NO2) and a reduced form of Nitrogen (NH3) with a subsequent condensative reaction to give N2 and H2O as principle reaction products. However in certain instances additional and competing processes may be initiated which can result in loss of reductant concentration i.e. so-called parasitic oxidation (eqns 7-9) of the injected Urea/Ammonia resulting in the formation of N2 and H2O (as a best case scenario—eqn 7), the generation of N2O, powerful Greenhouse gas (approximately 300 stronger than CO2 eqn 9), or even additional NOx (eqn 8).
(3)(NH2)CO + 4H2O → 2NH3 + 6CO2Urea hydrolysis(4) 4NO + 4NH3 + O2 → 4N2 + 6H2Ostandard/‘slow’ SCR(5) 3NO2 + 4NH3 → (7/2)N2 + 6H2ONO2 only SCR(6) NO + NO2 + 2NH3 → 2N2 + 3H2O‘Fast’ SCR(7) 4NH3 + 3O2 → 2N2 + 6H2Oparasitic NH3 oxidation to N2(8) 4NH3 + 5O2 → 4NO + 6H2Oparasitic NH3 oxidation to NO(9) 2 NH3 + 2O2 → N2O + 3H2Oparasitic NH3 oxidation to N2O
The principal reaction mechanism is represented in equation (3). However, under practical conditions it has been repeatedly demonstrated that the reaction of NO/NO2 mixtures with ca. 50% of the NOx present as NO2 results in the highest rate of NOx conversion by NH3 (eqn 4) (E. S. J. Lox Handbook of Heterogeneous Catalysis 2nd Edition, p 2274-2345 and references therein). Furthermore, while the reaction between NH3 and NO2 is known to occur (eqn 5), it is not kinetically dominant, hence as NO2 concentration increases above ca. 50% there is a concomitant decrease in catalyst activity and overall rates (A. Grossale, I. Nova, E. Tronconi, D. Chatterjee, M. Weibel, J. Catal, 256 (2008) 312-322). However, it should always be stressed that the rates of reactions will also vary greatly depending on the reaction temperature in this especial instance on the sort of the catalyst used and indeed upon the presence of reactive poisons in the gas stream and the relative poison tolerance of the different catalyst formulations employed therein.
NH3 SCR has been applied successfully for >20 years for the remediation of NOx from the exhaust gases of large industrial plants e.g. power stations. Hence there is extensive prior art in the field. The following discussion will attempt a brief précis of this body of work.
The first class of materials developed for the process is based upon vanadium oxide supported on titanium oxide. This class of catalyst may additionally be promoted by other metals such as Tungsten (or other acidic metals to enhance NH3 activation/adsorption) or Alkali or Alkaline Earth metal (as a NOx trap). Such technologies were initially developed for power stations but have more recently been applied to mobile applications. Their long commercial history, relatively low cost and high performance in the preferred operational window of ca. 200-400° C. makes this class of technology attractive for some applications. There are however several drawbacks to such catalysts which are especially severe for vehicular applications. These drawbacks include limited hydrothermal durability, especially under the rigorous conditions of DPF (diesel particulate filter) regeneration, limited catalyst lifetime, susceptibility to poisoning by exhaust components e.g. SOx, and poor activity at low (<250° C., NH3 activation and NO reduction is low) and higher temperatures (>ca. 400° C. parasitic oxidation of NH3 being problematic). Exemplary references for Vanadia-Titania SCR include U.S. Pat. No. 4,085,193, U.S. Pat. No. 4,916,107, U.S. Pat. No. 4,929,586, U.S. Pat. No. 5,827,489, U.S. Pat. No. 6,475,944, U.S. Pat. No. 7,431,895, U.S. Pat. No. 7,498,010 and US2005/0069477 A1 amongst others.
The second class of materials for SCR catalysts is based upon Zeolites. Zeolites are microporous crystalline aluminosilicate materials characterised by well ordered 3-D structures with uniform pore/channel/cage structures of 3 to 10 Angstroms (depending on framework type) and the ability to undergo ion exchange to enable the dispersion of catalytically active cations throughout the structure. Zeolites, metal exchanged Zeolites and promoted versions thereof have been studied in great detail for many years and provide highly active low and intermediate temperature SCR catalysts e.g. Japanese Patent 51-69476 (1976). Given the aforementioned flexibility in structure type and modification of Zeolites it is therefore unsurprising that an enormous body of papers and patents have accrued in this field. For example, U.S. Pat. No. 5,417,949 (Mobil 1995) describes a process for converting NOx with NH3 to N2/H2O under lean conditions using a molecular sieve having a Constraint Index of up to about 12, with the molecular sieve selected from the group having the structure of Zeolite Y, Zeolite L, Zeolite β, ZSM-4, ZSM-20, Mordenite, VPI-5, SAPO-11, SAPO-17, SAPO-34, SAPO-37, MCM-36, and MCM-41. Similarly there have been studies addressing the manipulation of the Silica:Alumina characteristics of the Zeolite to enhance activity and hydrothermal durability e.g. U.S. Pat. No. 7,118,722, U.S. Pat. No. 7,182,927. In addition there has been a considerable body of work examining the synthesis, characterisation and application of proton (U.S. Pat. No. 6,569,394 and U.S. Pat. No. 5,589,147) and Copper and Iron ion-exchanged Zeolites (U.S. Pat. No. 4,961,917, U.S. Pat. No. 6,843,971, U.S. Pat. No. 7,005,116 and U.S. Pat. No. 7,049,261). This large body of research has confirmed the high activity, broad temperature window and improved hydrothermal durability and poison tolerance of Zeolite systems cf. Vanadia-titania based SCR systems. However Zeolite SCR catalysts are not without drawbacks. For example extended or severe hydrothermal aging results in dealumination of the framework structure with a resultant loss of acidity (Y. Cheng, J. Hoard, C. Lambert, J-H. Kwakb and C. H. F. Peden, Catal Today 36 (1-2), (2008), 34-39). Moreover, HC retention in the Zeolite structure has been demonstrated as a limitation of conventional Zeolites due to accumulation of carbonaceous deposits and resultant active site blocking (Y. Huang, Y. Cheng and C. Lambert, SAE Int. J. of Fuels & Lubricants, vol. 1 (2009), 466-470). Additionally it has been demonstrated that combustion of retained HC, e.g. during post-injection, in ion-exchanged Zeolites can result in uncontrolled HC combustion and internal exotherm which steams and extracts cations from the Zeolite (J. Girard, R. Snow, G. Cavataio and C. Lambert, SAE 2008-01-0767).
More recently a new sub-class of Zeolite and Zeotype (structural isotypes/isomorphs based upon for example alumina-phosphate, silica-alumina-phosphate i.e. ALPO, SAPO) materials for SCR have been introduced. These materials are based upon so-called ‘8-ring’ structures of the structure type CHA (Chabazite) and related structure types e.g. AEI, AFT, AFX, DDR, ERI, ITE, ITW, KFI, LEV, LTA, PAU, RHO, and UFI. These alternate Zeolite structures show promise in addressing issues related to HC (hydrocarbon) uptake/site blocking and also for limiting deactivation by in-situ combustion, as the ‘critical-diameter’ of the Zeolites are so small that ingress of HCs into the internal porosity of the materials is limited, e.g. for CHA the channel diameters are 3.8*3.8 Å, thus only limited quantities of small HC molecules may enter. Moreover it has been found that both Chabazite (‘pure’ aluminosilicate) and SAPO-34 (silica-alumino-phosphate isomorph) display a surprisingly high hydrothermal durability and retain good activity after hydrothermal aging cycles as high as 900° C. (WO 2008/106519 A1 and WO 2008/118434 A1 for Chabazite and SAPO34 resp.). However, notwithstanding these significant improvements in durability and HC poisoning tolerance it should be highlighted that the these new Zeolites/Zeotypes still present common issues to all Zeolites in that they are comparatively expensive, time consuming to produce, and require specialised autoclaves operating at high pressure and temperature and have a somewhat limited supply base to serve the forthcoming volumes required to fulfil market requirements in the coming years.
In order to address the cost and supply concerns for Zeolite SCR catalysts there have been many efforts to develop simpler, robust mixed metal oxide catalyst systems of comparable efficacy. For example U.S. Pat. No. 5,552,128 describes the use of acidic solid Group IVB metal oxide modified with oxy-anion Group VIB metal and containing at least 1 metal ex Group IB, IVA, VB, VIIB, VIII and mixtures thereof, with Ni, Fe, Mn, Sn, Cu, Ru and mix Group IVB is Zr and Group VIB is W being especially favoured. More recently there have also been efforts to develop SCR catalysts based upon Zr—Si-Oxide, Zr—Si—W-Oxide and Zr—Ti—Si—W Oxides (WO/2008/046920, WO/2008/046921 and SAE 2007-01-0238). The use of TiO2-containing systems is also recorded in JP 52-42464 which cites a catalyst containing 50-97 (atomic %) titanium oxide as its first active ingredient, 2-49 (atomic % percent) cerium oxide as its second active ingredient, and 1-30% (atomic percent) of at least one compound selected from molybdenum oxide, tungsten oxide, vanadium oxide, iron oxide, and copper oxide as its third active ingredient with illustrative examples including Ti—Ce—Cu, Ti—Ce—Fe, Ti—Ce—W and Ti—Ce—Mo. Additionally WO/2008/150462 describes a complex multi-phase oxide catalyst system with high activity for NH3—SCR of NOx comprising at least two components wherein ‘the first component is selected from oxides of a transition metal other than the metal contained in the second component’, with V2O5, MoO3, WO3, and mixtures and combinations thereof being preferred. This active phase is present at 0.1% to 30% and is supported by a second component from oxides of Cerium or Lanthanide or Cerium/Lanthanide/Titanium/Zirconium or combinations and mixtures thereof. Further examples of oxide base SCR may be found in EP 1736232 which describes a complex oxide consisting of 2 or more oxides selected from silica, alumina, titania, zirconia, and tungsten oxide; and a rare earth metal or a transition metal except Cu, Co, Ni, Mn, Cr, and V. The use of Zirconium oxide support for SCR catalysts is also recorded in N. Apostolescu et al. in Appl Catal B: Env 62 (2006) 104-114, for a 1.4 mol % Fe and 7.0 mol % WO3 on ZrO2 catalyst. Similarly U.S. Pat. No. 5,552,128 cites Group IVB (Zr) metal oxide as a support with catalytic modification being provided by an oxyanion of a Group VIB metal (e.g. W) with further promotion by use of at least one metal selected from the group consisting of Group IB, Group IVA, Group VB, Group VIIB and Group VIII (Fe) and mixtures thereof. Additionally sulphated Zirconia may be employed as a support for an SCR catalyst, again in conjunction with specific transition metals e.g. tungsten or molybdenum oxide (JP 2003-326167). More recently US 2008/0095682 A1 proposes the use of composite oxides based upon Cerium Zirconium with additionally containing Mo/Mn W, Nb, Ta. Further examples may also be found in GB 1473883, WO/2008/085265 and WO 2009001131. However, in all cases the activity displayed by such systems remain somewhat below that observed by the preferred Zeolite systems, particularly after hydrothermal aging cycles.
Hence what is required in the art is a technology to provide highly active and selective SCR catalysis with improved hydrothermal durability and decreased cost. Additionally the new technology must provide the aforementioned improvements whilst retaining a wide operating range, tolerance to high NO2 contents and also possessing enhanced resistance to HC and SOx poisons present in the exhaust stream to fulfil the requirements of modern multi-brick emission control architectures.